Precision genetic control is an essential feature of living systems, as cells must respond to a multitude of biochemical signals and environmental cues by varying genetic expression patterns. Most known mechanisms of genetic control involve the use of protein factors that sense chemical or physical stimuli and then modulate gene expression by selectively interacting with the relevant DNA or messenger RNA sequence. Proteins can adopt complex shapes and carry out a variety of functions that permit living systems to sense accurately their chemical and physical environments. Protein factors that respond to metabolites typically act by binding DNA to modulate transcription initiation (e.g. the lac repressor protein; Matthews, K. S., and Nichols, J. C., 1998, Prog. Nucleic Acids Res. Mol. Biol. 58, 127-164) or by binding RNA to control either transcription termination (e.g. the PyrR protein; Switzer, R. L., et al., 1999, Prog. Nucleic Acids Res. Mol. Biol. 62, 329-367) or translation (e.g. the TRAP protein; Babitzke, P., and Gollnick, P., 2001, J. Bacteriol. 183, 5795-5802). Protein factors responds to environmental stimuli by various mechanisms such as allosteric modulation or post-translational modification, and are adept at exploiting these mechanisms to serve as highly responsive genetic switches (e.g. see Ptashne, M., and Gann, A. (2002). Genes and Signals. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.).
In addition to the widespread participation of protein factors in genetic control, it is also known that RNA can take an active role in genetic regulation. Recent studies have begun to reveal the substantial role that small non-coding RNAs play in selectively targeting mRNAs for destruction, which results in down-regulation of gene expression (e.g. see Hannon, G. J. 2002, Nature 418, 244-251 and references therein). This process of RNA interference takes advantage of the ability of short RNAs to recognize the intended mRNA target selectively via Watson-Crick base complementation, after which the bound mRNAs are destroyed by the action of proteins. RNAs are ideal agents for molecular recognition in this system because it is far easier to generate new target-specific RNA factors through evolutionary processes than it would be to generate protein factors with novel but highly specific RNA binding sites.
Although proteins fulfill most requirements that biology has for enzyme, receptor and structural functions, RNA also can serve in these capacities. For example, RNA has sufficient structural plasticity to form numerous ribozyme domains (Cech & Golden, Building a catalytic active site using only RNA. In: The RNA World R. F. Gesteland, T. R. Cech, J. F. Atkins, eds., pp.321-350 (1998); Breaker, In vitro selection of catalytic polynucleotides. Chem. Rev. 97, 371-390 (1997)) and receptor domains (Osborne & Ellington, Nucleic acid selection and the challenge of combinatorial chemistry. Chem. Rev. 97, 349-370 (1997); Hermann & Patel, Adaptive recognition by nucleic acid aptamers. Science 287, 820-825 (2000)) that exhibit considerable enzymatic power and precise molecular recognition. Furthermore, these activities can be combined to create allosteric ribozymes (Soukup & Breaker, Engineering precision RNA molecular switches. Proc. Natl. Acad. Sci. USA 96, 3584-3589 (1999); Seetharaman et al., Immobilized riboswitches for the analysis of complex chemical and biological mixtures. Nature Biotechnol. 19, 336-341 (2001)) that are selectively modulated by effector molecules.
These properties of RNA are consistent with speculation (Gold et al., From oligonucleotide shapes to genomic SELEX: novel biological regulatory loops. Proc. Natl. Acad. Sci. USA 94, 59-64 (1997); Gold et al., SELEX and the evolution of genomes. Curr. Opin. Gen. Dev. 7, 848-851 (1997); Nou & Kadner, Adenosylcobalamin inhibits ribosome binding to btuB RNA. Proc. Natl. Acad. Sci. USA 97, 7190-7195 (2000); Gelfand et al., A conserved RNA structure element involved in the regulation of bacterial riboflavin synthesis genes. Trends Gen. 15, 439-442 (1999); Miranda-Rios et al., A conserved RNA structure (thi box) is involved in regulation of thiamin biosynthetic gene expression in bacteria. Proc. Natl. Acad. Sci. USA 98, 9736-9741 (2001); Stormo & Ji, Do mRNAs act as direct sensors of small molecules to control their expression? Proc. Natl. Acad. Sci. USA 98, 9465-9467 (2001)) that certain mRNAs might employ allosteric mechanisms to provide genetic regulatory responses to the presence of specific metabolites. Although a thiamine pyrophosphate (TPP)-dependent sensor/regulatory protein had been proposed to participate in the control of thiamine biosynthetic genes (Webb & Downs, Characterization of thiL, encoding thiamin-monophosphate kinase, in Salmonella typhimurium. J. Biol. Chem. 272, 15702-15707 (1997)), no such protein factor has been shown to exist.
Transcription of the lysC gene of B. subtilis is repressed by high concentrations of lysine (Kochhar, S., and Paulus, H. 1996, Microbiol. 142:1635-1639; Mäder, U., et al., 2002, J. Bacteriol. 184:4288-4295; Patte, J. C. 1996. Biosynthesis of lysine and threonine. In: Escherichia coli and Salmonella: Cellular and Molecular Biology, F. C. Neidhardt, et al., eds., Vol. 1, pp. 528-541. ASM Press, Washington, D.C.; Patte, J.-C., et al., 1998, FEMS Microbiol. Lett. 169:165-170), but that no protein factor had been identified that served as the genetic regulator (Liao, H.-H., and Hseu, T.-H. 1998, FEMS Microbiol. Lett. 168:31-36). The lysC gene encodes aspartokinase II, which catalyzes the first step in the metabolic pathway that converts L-aspartic acid into L-lysine (Belitsky, B. R. 2002. Biosynthesis of amino acids of the glutamate and aspartate families, alanine, and polyamines. In: Bacillus subtilis and its Closest Relatives: from Genes to Cells. A. L. Sonenshein, J. A. Hoch, and R. Losick, eds., ASM Press, Washington, D.C.).